Seals are used in aircraft engines to isolate a fluid from one or more areas/regions of the engine. For example, seals control various parameters (e.g., temperature, pressure) within the areas/regions of the engine and ensure proper/efficient engine operation and stability.
Referring to FIGS. 2A-2B, a prior art sealing system 200 is shown. The system 200 is used to provide an interface between a static engine structure 206 and a rotating engine structure 212. The system 200 includes a floating, non-contact seal 218 that is formed from beams 230a and 230b and a shoe 236 coupled to the beams 230a and 230b. The seal 218 may interface to the structure 206 via a carrier 242. A spacer 248 may separate the carrier 242 and/or the beams 230a and 230b from a seal cover 254. Secondary seals 260 may be included in a cavity formed between the spacer 248, the cover 254 and the shoe 236. The spacer 248 and/or the seal cover 254 may help to maintain an (axial) position of the secondary seals 260. The shoe 236 may interface to (e.g., may slide or rotate with respect to) a scalloped plate 266. The shoe 236 and the beams 230a and 230b may interface to an outer ring structure 272. The seal 218 may include at least some characteristics that are common with a HALO® seal provided by, e.g., Advanced Technologies Group, Inc. of Stuart, Fla.
In operation, air flows from a high pressure area/region 270 of the engine to a low pressure area/region 280 of the engine as shown via the arrow 284. As the air flows passes teeth 238 of the shoe 236 (where the teeth 238 are frequently formed as thin knife-edges), an associated pressure field changes. This change induces the shoe 236 to move in, e.g., the radial reference direction until an equilibrium condition is obtained. In this respect, the seal 218 is adaptive to changing parameters and allows for maintenance of clearances between the structures 206 and 212 within a relatively tight range in order to promote engine performance/efficiency. The secondary seals 260 may promote the flow 284 from the high pressure region 270 to the low pressure region 280 as shown between the shoe 236 (e.g., teeth 238) and the rotating structure 212.
As part of conventional seal designs, to accommodate the movement of the shoe 236 described above the beams 230a and 230b are arranged so as to be substantially parallel to the shoe 236, creating an ‘S’-like shape 288 between the beams 230a and 230b and the shoe 236 as superimposed in FIG. 2B. And, as shown in FIG. 2C, a four-bar linkage 292 is created by the parallel beams 230a and 230b. 
Conventional seal designs, such as those shown in relation to FIGS. 2A-2C, may be prone to beam imbalance. For example, and as shown in FIG. 2C, if a first portion of the shoe 236 at a first location 296a (e.g., a first circumferential location) moves a first distance (e.g., a first radial distance) and a second portion of the shoe 236 at a second location 296b (e.g., a second circumferential location) moves a second distance (e.g., a second radial distance) that is different from the first distance, the shoe 236 may be prone to “saw-toothing”. If this saw-toothing/imbalance in terms of the differences in movement/deflection between the first location 296a and the second location 296b is substantial enough, the gap/distance control that is sought by the use of the adaptive seal 218 may be at least partially defeated, which may result in excessive rubbing and/or excessive leakage. Additionally, since the beams 230a and 230b are relatively crowded in the conventional, parallel configuration, space is limited and the length of the beams 230a and 230b is limited. This limitation in the length of the beams 230a and 230b limits the range of the travel of the shoe 236.